1. Field of the Invention
The present invention relates to cling films and the like, and in particular to tackifiers for such films. More particularly the invention relates to improved polyolefin cling films which incorporate enhanced polyisobutylene (PIB) tackifiers. The invention also relates to methods for improving the cling characteristics of cling films by incorporating therein enhanced PIB tackifiers.
2. The Prior Art Background
Cling film is a well known product having many industrial uses including, for example, and without limitation, use as an over wrap for pallets and as a wrap for silage. The clinging properties of cling film are often improved and enhanced by the incorporation therein of a tackifier. Tackifiers are often incorporated into the cling film by adding the same to the melted resin during the production of the film.
Generally speaking, the base structure of a cling film may be any one of a number of polyolefin materials. For example, polypropylene may also be used as a cling film in high performance applications requiring an extra sturdy wrap. However, in common practical commercial usage, the base film usually comprises a polyethylene. There are several classes of polyethylene compositions which are suitable for such purposes, including high density polyethylene (HDPE), low density polyethylene (LDPE) and linear low density polyethylene (LLDPE).
One type of polyethylene that may be used to make cling film is a Dow Chemical LLDPE which incorporates an octene-1 comonomer. Other types of LLDPE which are often used in cling film applications include those made by Exxon, British Petroleum and the former Union Carbide (now merged with Dow). These LLDPE materials often incorporate comonomers such as butene-1 or hexene-1. In these materials, it is common for the base film to include from about 9 to about 11 weight percent of the comonomer.
As mentioned above, the clinging properties of cling film are often improved and enhanced by the incorporation therein of a tackifier. Conventionally, when the base film comprises LDPE or LLDPE, conventional polybutene is most often used as a tackifier. As with other known tackifiers, the conventional polybutene tackifier is simply added to the resin melt during the production of the cling film. Well known methods for producing films, and particularly cling films, include processes where the film is blown and processes where the film is extruded.
Conventional polybutene products of the sort which have previously been used as tackifiers for cling film are described, for example, in German patents nos. DE 19619267 dated Nov. 20, 1997 and DE 19520078 dated Dec. 12, 1996, and in U.S. Pat. Nos. 5,068,490 dated Nov. 26, 1991, 4,605,808 dated Aug. 12, 1986 and 5,286,823 dated Jun. 22, 1992. These prior patents are all directed to related subject matter. Generally speaking, these known polybutene products may range in number average molecular weight (MN) from 500 to 1,000,000 or higher, and generally the same have a single double bond per molecule. The double bonds of these previously known polybutene materials are mostly internal. That is to say, it is normal for at least about 90% of the double bonds to be at an internal position and for less than about 10% of the double bonds to be in a terminal position. In addition, while the initial monomeric mixture used to produce these conventional polybutenes is predominantly isobutylene, the same usually incorporates substantial quantities of monomers other than isobutylene. Thus, the conventional polybutenes used as tackifiers may generally be referred to as copolymers or terpolymers. For example, conventional polybutenes often and commonly incorporate about 10% or so by weight of 1-butene and/or 2-butene, with only about 90% of the initial monomeric mix being isobutylene.
The polybutene tackifier is introduced into the resin melt in the film extruder or other apparatus used to produce the film and the same is therefore blended with the base polymer mix being used in the production of the film. When the polybutene tackifier is thus compounded into the blown or extruded films, it initially is randomly dispersed throughout the entire body of the produced film. Polybutene is generally incompatible with the materials used as the base structure of the film, and the same will therefore slowly migrate to the surface of the film where it is active in promoting the cling properties of the film because its inherent stickiness provides tack for the film. The migration of the polybutene to the surface of the film is referred to as blooming, and the rate at which migration to the surface occurs is referred to as the bloom rate. The amount of time required for this migration to occur so as to achieve optimum cling properties is known as bloom time.
As discussed above, conventional polybutene has been used in the past as a tackifier for plastic films. But such use has not been without it own set of problems. Current problems encountered when conventional polybutene is used in cling film applications include difficulty in handling due to the high viscosity of the polybutene material, inconsistent bloom rates due to the variability of the polybutene tackifier material, bloom times that are too long, smoking during heating and processing, high noise levels in machine wrap applications, condensation on air rings, and loss of polybutene during processing.
In accordance with the concepts and principles of the present invention a new and highly useful cling film tackifier is provided in the form of an enhanced PIB polymer product which is comprised essentially of a homopolymer of isobutylene wherein the double bonds are mainly in the alpha position. In further accordance with the preferred aspects of the invention, the PIB polymer product desirably may have a polydispersity no greater than about 2.2 and an MN that is within the inclusive range of from about 900 to about 3000.
Preferably, in farther accordance with the concepts and principles of the invention, at least about 95%, desirably at least about 97%, and ideally, at least about 99% of the monomeric units incorporated in the PIB polymeric tackifier molecules are isobutylene moieties. Even more desirably, no more than about 1% of the monomeric units incorporated in the PIB polymeric tackifier molecules are something other than isobutylene moieties.
In still further accord with the concepts and principles of the invention, at least about 40%, preferably at least about 50%, even more preferably at least about 60%, desirably at least about 70%, more desirably at least about 80%, even more desirably at least about 90%, and ideally more than 90% of the double bonds of the PIB polymeric tackifier product of the invention may be in the alpha position. In a most preferred form of the invention, no more than about 1% of the double bonds of the PIB polymer product are in a position other than alpha or beta.
The invention further provides an improved cling film comprising a PIB tackifier as outlined above. Broadly, the improved cling film of the invention may desirably have a polyolefin base structure. Ideally, the cling film of the invention may have a polyethylene base structure. Desirably, the cling film of the invention may include from about 1.5 to about 8.0% by weight of the PIB tackifier. Ideally, the cling film of the invention may include from about 3.0 to about 4.0% by weight of the PIB tackifier.
The invention also provides a method for improving the cling properties of a cling film which comprises including a PIB tackifier as outlined above in the cling film.